1. Field of the Invention:
The present invention relates to a semiconductor device having a Schottky barrier, and more particularly to a semiconductor integrated circuit device having a Schottky barrier diode formed between a metal silicide layer and a silicon layer.
2. Description of the Prior Art:
A Schottky barrier diode is often connected between the base and collector of a transistor in order to suppress an excessive lowering (or rise) in the collector potential in its conductive state. The switching characteristics of the transistor are thereby improved. There are cases where a Schottky barrier junction is employed as the base-emitter junction or the base-collector junction of a transistor, or the gate junction of a junction type field-effect transistor.
A Schottlky barrier used for such purposes is often constituted by a junction formed between a metal silicide layer and a silicon layer in a semiconductor integrated circuit device. The metal silicide layer is provided in such a manner that a metal is selectively deposited on a silicon layer and is then heat-treated. In an integrated circuit device, metal wiring layers are employed for interconnections between a Schottky barrier diode and a transistor, or between a transistor element having a Schottky barrier junction and another or other circuit elements. In other words, the metal wiring layer is in direct contact with the metal silicide layer. For this reason, the heat treatment which is carried out after the formation of the metal wiring layers may cause a metal contained in the metal wiring layer to diffuse into the metal silicide layer, resulting in a destruction of the Schottky barrier. Even if the destruction of the Schottky barrier does not occur, the diffusion of the metal into the metal silicide layer brings about various kinds of deterioration in electrical characteristics, such as lowering in a forward voltage, a reduction in a breakdown voltage and an increase in a leakage current.
In order to prevent the diffusion of the metal into the metal silicide layer, it has been proposed that a diffusion barrier layer made of a metal, such as chromium, titanium or vanadium, is formed between the metal silicide layer and the metal wiring layer to prevent the direct contact between them. It is, however, extremely difficult to form the diffusion barrier metal layer with a uniform film thickness. In consequence, any portion of the diffusion barrier metal layer which is thin undesirably allows the metal which constitutes the metal wiring layer to diffuse into the metal silicide layer. If the diffusion barrier layer having a large thickness is employed in order to obtain a satisfactory barrier action, then a thermal stress is applied to the metal silicide layer due to the difference in the thermal expansion coefficient between the diffusion barrier layer and the metallic silicide layer, resulting in the deterioration of the electrical characteristics. Moreover, an additional step is required for forming the diffusion barrier layer.